Water security is a prerequisite to achieve food security. Water harvesting offers under-exploited opportunities for the drylands and the predominantly rainfed farming systems of the developing world. The principle is simple: capture potentially damaging rainfall runoff and translate this into plant growth or water supply. This makes clear sense where rainfall is limited, uneven or unreliable with pronounced dry spells. These new guidelines introduce the concepts behind water harvesting and propose a harmonised classification system, followed by an assessment of suitability, adoption and up-scaling of practices